Recently, polycarbonates have come into extensive use in fields such as optical disks, lenses, optical waveguides, and the like. In the fields of conventional general articles the, use of polycarbonate resins in producing precision molded articles having smaller wall thicknesses is increasing based on the heat resistance, transparency, safety, dimensional stability, mechanical strength, and other properties possessed by such resins.
Of those purposes of use, where transparency is an essential requirement, such as in optical applications in particular, molded articles are prevented from exhibiting their desired functions by dust particles, i.e., contaminants, contained in the articles.
For this reason, many studies have been made to lower the dust particle content in raw material polycarbonate resins. However, the difference in contamination caused in injection molding due to differences in materials among the injection molding machines has not received much attention, and there has been a pronounced tendency to believe that the burning, metal impurities, and other contaminants present in molded articles are attributable to differences in the raw material polycarbonates in heat resistance, etc.
In conventional injection molding, raw polycarbonate resins have been contaminated with a large amount of dust particles and the injection molding conditions have been considerably mild as compared to those for recent purposes of use. Hence, dust particle formation during injection molding operations has been negligible in most cases. Resin manufacturers, therefore, have been first required to make raw polycarbonates clean.
Injection molding machines have been suitably selected by raw resin manufacturers or users of the raw resins from various kinds of injection molding machines provided by manufacturers thereof, and the raw resin manufacturers and the resin users have been unable to afford to consider details of their injection molding machines, or there has conventionally been no necessity of doing so. Further, ordinary injection molding machine manufacturers have had no knowledge of the fact that the dust particle contents in injection-molded articles vary depending on the materials used to manufacture the molding machine. Furthermore, it is difficult to say that recent molding processes for producing these molded articles which are being conducted under exceedingly severe conditions as compared to conventional processes have been properly understood.
On the other hand, it has been known from experience that carbon steels catalytically promote gelation of polycarbonate resins. The present inventors have ascertained that when a molten polycarbonate resin is brought into contact with SACM steel (Al-Cr-Mo steel), which is a low-alloy steel and is one of the ordinary carbon steels, browning of the resin occurs on the surface of the steel and, in extreme cases, this brown resin turns black. Since this brown or black substance was insoluble in halogenated hydrocarbons that are a good solvent for polycarbonate resins, it has been presumed that this product is a gelation product formed through the decomposition, network formation, and even carbonization of the polycarbonate resin. Such products are formed in large quantities even when molding is conducted in dust particle-controlled clean atmospheres where raw polycarbonate resins are fed in a completely sealed state.
It is clear from the above that most of the dust particles contained in conventional low-dust-level polycarbonate molded articles were newly formed in the injection molding machine when the solid polycarbonate was remelted or plasticized and then injected. It can, therefore, be presumed that in order to produce polycarbonate molded articles having a low dust level, use of a special molding machine, particularly use of specific kinds of materials which result in little formation of dust particles, is essential.
However, materials which are usable for producing cylinders, screws, and other parts of injection molding machines which cause little dust particle formation, and can be fabricated economically, have not yet been found.